The close contact between two phases such as a gas and a liquid to promote their chemical or physical interaction is an important operation in chemical engineering.
To promote interface phenomena on the contact surface between these two phases, it is endeavoured to increase this contact surface as much as possible, and to increase the effects of mixing in the vicinity thereof.
For such purpose, beds of fine solid particles are frequently used through which a fluid passes and with which they interact.
These beds, called particle beds or packings, offer large exchange surfaces on account of the small size of their constituent particles, and on account of the large divided status of the fluid passing through them.
These phenomena promote the speedy accomplishing of material transfer processes, chemical reactions or any other diffusion-related phenomena.
Their applications particularly cover the fields of both analytical and preparative liquid and gas chromatography.
U.S. Pat. No. 4,657,742 to Beaver P. proposes an alternative to these particle packings comprising a tube packed with aligned fibres which may be porous and hollow. One disadvantage of this packing is that the eluting fluid flows both inside and outside the hollow fibres in the voids left between their stacking. Since the eluting fluid flows at two very different rates inside the hollow fibres and on periphery thereof in the interstices separating the fibres of circular section, there is resulting loss of efficacy. Another disadvantage of this device is that the walls of the hollow fibres must be sufficiently thick so that they can be handled and packed withstanding the mechanical stresses induced by the stacking thereof. This means that the diffusional balancing between adjacent fibres is slow, and the packing is little efficient. Another disadvantage of this device is that it is difficult to apply to bundles of fibres of large diameter since the chemical stability of the packing would be difficult to ensure.
Patent U.S. Pat. No. 4,957,620 to Cussler E. describes the use of bundles of hollow polymer fibres for use as chromatographic column. The assembly suffers from the same disadvantages as above: the thickness of the wall of the fibres must be higher than that of the central channel in order to impart sufficient mechanical strength to these fibres allowing the handling and assembly thereof. As a result, transfers of material by diffusion between the material of the walls and eluting fluent are slow. The eluting fluid flows at two very different rates inside the hollow fibres and on the periphery thereof. Here again the stabilization of large diameter packing is difficult owing to the lack of strong bonds between adjacent fibbers.
Patent U.S. Pat. No. 4,818,264 describes the use of bundles of capillary columns in glass or silica to perform multicapillary gas chromatography. This system has the serious drawback that the capillaries behave independently of each other. On this account, it is difficult to obtain identical behaviour of the different channels and careful, scrupulous attention must be given to the manufacture of channels that are all identical.
Patent application US 2005/0139536 to Belov Y. P. describes a chromatographic column whose channels are coated with different thicknesses of stationary phase so as to offset hydrodynamic inequalities between the different channels. This work exemplifies the difficulty in obtaining good performance levels with a multicapillary column formed of individualized channels which do not communicate by diffusion.
The publications by Nishihara H. «Ordered macroporous silica by ice templating», Chemistry of Materials, 28 Feb. 2005, pages 683-689 and Mukai S. R. «Formation of monolithic silica gel microhoneycomb (SMH's) using pseudo steady state growth of microstructural ice crystals» Chemical Communications, 4 Mar. 2004, pages 874-875 describe a potential pathway for forming multicapillary structures in silica. The documents refer to a method of manufacturing microstructures of ordered porous silica, of honeycomb shape and 3.6 to 40 μm in diameter. The method comprises causing directional growth of ice crystals in low-cohesion silica gels and evaporating a solvent by freeze-drying.
However, the described method only functions with silica gels having low cohesion i.e. with low silica concentration. The structures obtained are therefore very lightweight, namely having a density of the order of 0.12 g/cm3 according to the authors of these publications. The relative volume of the capillaries is high. As such, they will not perform well in liquid chromatography for which a dense packing is sought having strong retention capacity. In addition, packing that is so lightweight is mechanically fragile.
Additionally, examination of all the photographs in the two articles shows that the diameters of the channels differ by a factor of about 10, and that these channel diameters fluctuate to a large extent and are irregular over their length. These channels have most variable environment and morphology, their cross-section possibly being square, pentagonal, hexagonal, etc. These irregularities mean that such packing is inefficient for high performance analytical chromatography for which perfect homogeneity of the packing is required.
Finally, the packings described in these articles are obtained over a restricted range of diameters, from 3.6 to 42 μm. Yet, the range extending below 2 μm is of particular interest for application in high pressure liquid chromatography (HPLC), and the range extending above 50 μm is of particular interest for application to gas chromatography.
Patent U.S. Pat. No. 6,210,570 to Holloway R. describes monolithic packing in porous silica for chromatography. Said packing is formed of more or less spherical pores forming tortuous passages through the packing. These passages are tortuous and a fluid passing them encounters numerous obstacles, the pores and the solid being randomly distributed in space within the packing. This forms a major difference with a flow through an empty capillary tube in which the fluid does not encounter any microscopic obstacle over an optimal rectilinear pathway. They display a lower pressure drop than a particle packing but higher than that of a capillary having the same separation efficacy for a given analysis, and have intermediate separation impedance between the two. They have the advantage of allowing a macroscopically uniform flow of the eluting fluid through the packing on account of their monolithic structure, unlike the case with the stack of capillary tubes described in U.S. Pat. No. 4,657,742.
The following publications: N, Ishizuka, Designing monolithic double pore silica for high speed liquid chromatography, Journal of Chromatography A, 797 (1998), 133-137, K Nakanishi, Phase separation in silica sol-gel system containing polyacrylic acid, Journal of non crystalline Solids 139 (1992, 1-13 and 14-24, K. Nakanishi, Phase separation in Gelling Silica-Organic Polymer Solution: Systems Containing Poly(sodium styrenesulfonate), J. Am. Ceram. Soc. 74 (10) 2518-2530-30 (1991) deal with the same subject matter as the Holloway patent. The aim is to obtain a monolithic packing in silica comprising two families of pores, one of interconnected macropores in which a liquid is able to flow relatively freely and the other a family of mesopores or micropores creating a specific surface area and hence activity for the exchange of material.
However, the large majority of separations are still conducted on particle beds, which are easier to manufacture.
There is therefore a need for a product having advantages in terms of reliability and ease of manufacture of particle packing, allowing the uniform microscopic and macroscopic flow of eluting fluid in the bed, whilst maintaining the advantages of capillary columns.